Non-contact electro-optical measuring systems are advantageous over those using mechanical sensing, because of their relatively high speed and non-destructive capabilities. The increasing need for non-contact surface displacement and profile measurements has already led to the development of such electro-optical systems as those incorporating interferometry methods, speckle detection, Moire deflectometry, stereo vision, focus error detection, time of flight, confocal microscopy and structured light triangulation.
Confocal microscopy and structured light triangulation are widely known methods for measuring surface profiles or displacements. Scanning confocal microscopes can measure surface profiles with high accuracy and in on-axis configuration which minimizes shadowing problems. However, the measurements are performed serially (point by point) and are therefore usually very slow.
The structured light triangulation methods are the most wide-spread. They are suitable for industrial applications, in that they offer a simple and robust 3-D measurement. Structured light triangulation systems determine distance to an object by projecting thereon light from a source and imaging the projected pattern on to a detector. With the position of the image on the detector, the lateral separation between the detector lens and the light source, and the projection angle of the source being known, the distance to the object is determined. Sequential measurements at different coordinates on the object lead to a full 3-D image of the object's surface profile.
The simplest structured light system projects a single point of light from the light source on to an object. The point is then imaged on a detector which is in the form of lateral effect photodiodes or a linear array. The imaging is performed point by point until the surface is scanned completely. The 3-D measurement according to this procedure can be inexpensive and has high resolution. The single point triangulation system, however, involves lengthy time-consuming scanning, which is often impractical.
A second class of triangulation systems operates by projecting a light stripe on to an object and using a two-dimensional detector array. Fewer frames are needed to scan over an object, and scanning need only be done in the direction perpendicular to the stripe.
However, the above light triangulation systems cannot simultaneously achieve a large depth measuring range and high lateral resolution. The reason for this is that usually, in optical elements, a long focal depth cannot be combined with a small spot size since the latter requires high numerical apertures, whereas the former requires low numerical apertures. Mathematically, the combination of Abbe's formula for lateral resolution ##EQU1## where .delta.x is the spot size, and Rayleigh's formula for the depth of focus .delta.F yields: ##EQU2## where .lambda..sub.0 is the wavelength of the light and .kappa. is a constant number between 1 and 4 which depends on the exact definitions of .delta.x and .delta.F.
A common trade-off approach is to use a relatively large spot, the center of which can be determined with much higher accuracy than the spot-size, by complicated numerical techniques. Such an approach suffers from the drawback that it is highly sensitive to local changes in the reflectivity or the shape of the object. Thus, for example, short radii of curvature cannot be adequately dealt with using this approach.